A non-aqueous secondary battery represented by a lithium ion secondary battery has been widely used as a main power source of a notebook electronic apparatus such as a mobile phone or a notebook computer. The non-aqueous secondary battery is widely applied to a main power source of an electric vehicle or a hybrid car, an electricity storage system of night-time electricity, and the like. With the spread of the non-aqueous secondary battery, it is necessary to ensure stable battery characteristics and safety.
In general, a porous membrane having polyolefin such as polyethylene or polypropylene as a main component is used as a non-aqueous-secondary-battery separator. However, when the battery including the polyolefin porous membrane is exposed at a high temperature, the separator may be melted down and this may cause smoking, ignition, or explosion of the battery. Accordingly, there is a demand for heat resistance in order to prevent the melt down of the separator at a high temperature.
In such viewpoints, in the related art, a separator formed by coating a heat-resistant porous layer including a heat-resistant polymer and ceramic particles on one surface of a polyethylene porous membrane is known (for example, PTL 1 and PTL 2). However, in a configuration of coating and forming the heat-resistant porous layer on a porous base material, ionic permeability of a composite membrane may be significantly deteriorated when the porous structure in the layer is not appropriately formed, compared to a case of a porous base material single body.
In recent years, an outer package of the non-aqueous-secondary-battery has been simplified according to miniaturization and light weight of the portable electronic apparatus. A stainless steel battery can has been initially used as the outer package, but an outer package of an aluminum can has been developed, and currently, a soft outer package of an aluminum laminate package has been also developed. In a case of using the aluminum laminate soft outer package, the outer package is flexible, and accordingly, a space may be formed between an electrode and a separator due to charging and discharging, and this may lead to a technical problem of a short cycle life. In order to solve such a problem, a technology of adhering the electrode and the separator is important and a number of technical proposals have been made.
As one proposal, a technology of using a separator in which a porous layer (hereinafter, also referred to as an adhesive porous layer) formed of a polyvinylidene fluoride type resin is laminated on a polyolefin microporous membrane which is a separator of the related art, has been known (for example, see PTL 3). However, in PTL 3, the polyethylene microporous membrane is mainly used as a porous base material and no specific investigation has been made regarding the improvement of heat resistance of the porous base material.
Meanwhile, PTL 4 discloses a separator in which a heat-resistant porous layer is provided on both surfaces of a porous base material and an adhesive porous layer such as a polyvinylidene fluoride type resin is further provided on the heat-resistant porous layer, and both of heat resistance and adhesiveness with an electrode are realized.